Typically, gas turbine engines are formed from a combustor positioned upstream from a turbine blade assembly. The turbine blade assembly is formed from a plurality of turbine blade stages coupled to discs that are capable of rotating about a longitudinal axis. Each turbine blade stage is formed from a plurality of blades extending radially about the circumference of the disc. Each stage is spaced apart from each other a sufficient distance to allow turbine vanes to be positioned between each stage. The turbine vanes are typically coupled to the shroud and remain stationary during operation of the turbine engine.
The tips of the turbine blades are located in close proximity to an inner surface of the shroud of the turbine engine. There typically exists a gap between the blade tips and the shroud of the turbine engine so that the blades may rotate without striking the shroud. During operation, high temperature and high pressure gases pass the turbine blades and cause the blades and disc to rotate. These gases also heat the shroud and blades and discs to which they are attached causing each to expand due to thermal expansion. After the turbine engine has been operating at full load conditions for a period of time, the components reach a maximum operating condition at which maximum thermal expansion occurs. In this state, it is desirable that the gap between the blade tips and the shroud of the turbine engine be as small as possible to limit leakage past the blade tips.
However, reducing the gap cannot be accomplished by simply positioning the components so that the gap is minimal under full load conditions because the configuration of the components forming the gap must account for emergency shutdown conditions in which the shroud, having less mass than the turbine blade and disc assembly, cools faster than the turbine blade assembly. In emergency shutdown conditions, the diameter of the shroud reduces at a faster rate than the length of the turbine blades. Therefore, unless the components have been positioned so that a sufficient gap has been established between the turbine blades and the turbine shroud under operating conditions, the turbine blades may strike the stationary shroud because the diameter of components of the shroud is reduced at a faster rate than the turbine blades. Collision of the turbine blades and the shroud often causes severe blade tip rubs and may result in damage. Thus, a need exists for a system for reducing gaps between turbine blade tips and a surrounding shroud under full load operating conditions while accounting for necessary clearance under emergency shutdown conditions.